A polymer electrolyte fuel cell in which fuel gas containing hydrogen is supplied to an anode while oxidizing gas containing oxygen is supplied to a cathode to cause electromotive force to be produced from an electrochemical reaction taking place at the two poles commonly consists mainly of a bipolar plate, gas diffusion layer, catalyst layer, electrolyte membrane, catalyst layer, gas diffusion layer, and bipolar plate stacked in this order. The gas diffusion layers are required to have a high gas diffusivity so that gas supplied from the bipolar plates are allowed to diffuse into the catalyst, high water removal performance so that water resulting from the electrochemical reaction is discharged into the bipolar plates, and high electrical conductivity so that the generated electric currents are taken out efficiently. Gas diffusion layers formed of carbon fiber and the like (hereinafter referred to simply as gas diffusion layer) have been used widely.
In a fuel cell, gas diffusivity and water removal performance should be high in the through-plane direction of the gas diffusion layers to ensure high fuel cell performance in a wide range of environment conditions. To meet this requirement, Japanese Unexamined Patent Publication (Kokai) No. 2010-192379 proposes a method of reducing the bulk density of the porous carbon fiber base substrate and Japanese Unexamined Patent Publication (Kokai) No. 2005-149745 proposes a method of reducing the thickness of the porous carbon fiber base substrate. To allow the gas (hydrogen and oxygen) coming from the bipolar plates to be supplied uniformly into the catalyst layers, on the other hand, gas permeability in the in-plane direction (in-plane gas permeability) of the gas diffusion layers should be reduced to depress short cuts of gas between the channels in the bipolar plates. To meet this requirement, Japanese Unexamined Patent Publication (Kokai) No. 2003-288906 proposes a method of increasing the bulk density of the porous carbon fiber base substrate and Japanese Unexamined Patent Publication (Kokai) No. 2008-127661 proposes a method of impregnating the porous carbon fiber base substrate with a porous layer. However, a trade-off relationship exists between increasing gas diffusivity and water removal performance in the through-plane direction and decreasing gas permeability in the in-plane direction. It has thus been difficult to realize both objectives in a higher level balance. Japanese Unexamined Patent Publication (Kokai) No. 2006-004787, furthermore, proposes a method of intentionally forming concave-shape portions in the porous layer surface to increase gas diffusivity in the through-plane direction, although leading to problems with the durability of the porous layer taking place when used repeatedly for power generation.
It could therefore be helpful to provide a gas diffusion layer that develops high fuel cell performance by increasing gas diffusivity and water removal performance in the through-plane direction and also depresses short cuts of gas between the channels in the bipolar plates by decreasing gas permeability in the in-plane direction and further aims to provide a gas diffusion layer that contains a porous layer with a highly smooth surface.